In charge generating elements, incident light induces a charge separation across various layers of a multiple layer device. In an electrophotographic charge generating element, also referred to herein as an electrophotographic element, under the influence of an applied field, electron-hole pairs produced within a charge generating layer separate and move in opposite directions to reduce the potential between an electrically conductive layer and an opposite surface of the element. The surface charge forms a pattern of electrostatic potential, also referred to as an electrostatic latent image. The electrostatic latent image can be formed by a variety of means such as, for example, imagewise radiation-induced discharge of a uniform potential previously formed on the surface. Typically, the electrostatic latent image is developed by contacting it with an electrographic developer to form a toner image, which is then fused to a receiver. If desired, the latent image can be transferred to another surface before development, or the toner image can be transferred before fusing.
Electrophotographic elements can be of various types, including both those commonly referred to as single layer or single-active-layer elements and those referred to as multiactive layer or multiple-active-layer elements. Single-active-layer and multiactive layer elements and their preparation and use are described in, for example, U.S. Pat. Nos. 4,701,396; 4,666,802; 4,578,334; 4,719,163; 4,175,960; 4,514,481; and 3,615,414, the disclosures of which are incorporated herein by reference.
Single layer elements contain, in addition to an electrically conductive layer, a single photoconductive layer that is active both to generate and transport charges in response to exposure to actinic radiation. Multiactive elements contain, besides an electrically conductive layer, at least two active layers, at least one of these layers being capable of generating charge, i.e., electron/hole pairs, in response to exposure to actinic radiation, referred to as a charge-generation layer (CGL), and at least one layer capable of accepting and transporting charges generated by the CGL, referred to as a charge-transport layer (CTL). In a multiactive element, either the CGL or the CTL is in electrical contact with both the electrically conductive layer and the remaining CTL or CGL. The CGL contains a charge-generation material, the CTL a charge-transport agent. One or both the CGL and CTL may further include a polymeric binder. Multiactive elements may also include other layers such as, for example, adhesive interlayers, protective overcoats, charge blocking layers, and the like.
Stabilization of an electrophotographic element to the effects of ultraviolet and/or short wavelength blue light by the inclusion of additives and/or binder polymers that strongly absorb ultraviolet and short wavelength blue light has been described in, for example, U.S. Pat. Nos. 4,869,986 and 4,869,987. Absorption of the undesired light by the stabilizing materials prevents absorption by the CTL transport material and inhibits the undesired transport material photochemistry. The use of particular polyesters as binders in the CTL is also effective in mitigating the deleterious effects of exposure to ultraviolet radiation. Suitable binder polymers are described in U.S. Pat. Nos. 4,840,860; 4,840,861; 5,112,935; 5,135,828; and 5,190,840, the disclosures of which are incorporated herein by reference.
In addition to the just-mentioned ultraviolet and short-blue radiation-induced photofatigue, an electrophotographic element exposed to office lighting or other relatively high intensity light sources may undergo undesirable changes in electrophotographic characteristics, for example, increased dark decay of the surface potential and increased residual potential caused by electrophotographic cycling. Increased dark decay, which can produce nonuniformities in the subsequent toned image, may be due to the presence of dissolved dye or pigment charge generation material in the CGL, the CTL, or in the CGL/CTL interfacial region. The insoluble form of the pigment comprising the charge generation material can be solubilized during the solvent coating of the CTL over the CGL. The solubilized CGL material will have significant absorption in the visible region in the spectrum, and its presence, even in minute amounts, may cause an increased rate of dark discharge of the surface potential. The present invention, by decreasing the sensitivity of a photoreceptor to photofatigue induced by exposure to visible light, provides an effective solution to this serious problem.